1. Field of the Invention
This invention relates to lasers, and more particularly to ridge-waveguide and buried-heterostructure lasers.
2. Description of the Related Art
Lasers with low threshold currents, high output powers in a single spatial mode and large modulation bandwidths are required for optical communications applications. In addition, it is often desirable to monolithically integrate the lasers on the same substrate with electronic signal processing and driver circuitry.
One type of device that has been of interest for this purpose is the ridge-waveguide laser. These devices were reported in I. P. Kaminow et al., "Low-Threshold InGaAsP Ridge Waveguide Lasers at 1.3 .mu.m", IEEE Journal of Quantum Electronics, Vol. QE-19, No. 8, pages 1312-1318, August 1983, and are now quite common.
A ridge-waveguide laser is illustrated in FIG. 1. The device is fabricated on a semiconductor substrate 2 such as InP. A ridge 4 is etched over the substrate for current channeling and optical-mode confinement in the laser. The optical mode is indicated by the dashed line circle 5. With the earlier devices, a generally homogeneous active layer 6 of lasing material, such as InGaAsP, is buried beneath the ridge and extends laterally to either side of the ridge. An upper cladding layer 8 is located above the active layer 6 and includes the ridge 4, while a lower cladding layer 10 separates the active layer 6 from the substrate 2. The two cladding layers and the ridge are formed from a material having a higher bandgap energy than in the active layer 6, and accordingly work to confine charge carriers in the active layer. The cladding material could also be InGaAsP, but with a different percentage mix of the various components than in the active layer to provide the cladding layers with the required greater bandgap energy. For the ridge to successfully perform its current channeling and optical-mode confinement functions, the upper cladding layer 8 between the ridge and the active layer is quite thin, generally on the order of 0.2-0.3 microns or less. Lateral current spreading into the regions on either side of the ridge is reduced by the thin cladding layers, while optical-mode confinement is obtained by the effective refractive index step due to the ridge.
More recently, ridge-waveguide lasers have used multiple quantum wells (MQWs) or single quantum wells (SQWs) as the active lasing medium instead of a thicker homogeneous active medium. These types of devices are discussed in O. Wada et al, "Very Low Threshold Current Ridge-Waveguide AlGaAs/GaAs Single-Quantum-Well Lasers", Electronics Letters, Vol. 21, No. 22, Oct. 24, 1985, pages 1025-1026, and H. D. Wolf et al., "High-Speed AlGaAs/GaAs Multiple Quantum Well Ridge Waveguide Lasers", Electronics Letters, Vol. 25, No. 13, Aug. 31, 1989, pages 1245-1246.
Ridge-waveguide lasers have yielded reliable performance and their fabrication is relatively simple; the lasers can be fabricated together with other electronic devices on the same chip. However, the performance of ridge-waveguide lasers is limited by current spreading in the active area lateral to the ridge. Such current spreading is indicated by arrows 12 in FIG. 1. In addition to this current spreading and a consequent loss of power and efficiency, ridge-waveguide lasers are limited to a modulation bandwidth less than 15 GHz, whereas certain radar and communications applications require higher modulation rates.
Another approach to lasers for communications applications is the buried-heterostructure laser. In these lasers, the active region is surrounded in both the vertical and lateral directions by cladding material of greater bandgap energy. The lateral cladding material is typically formed in a separate material growth step. However, it can also be formed from the originally grown material by compositional disordering. Compositionally disordered buried heterostructure lasers are discussed in K. Meehan et al., "Stripe-Geometry AlGaAs-GaAs Quantum-Well Heterostructure Lasers Defined by Impurity-Induced Layer Disordering", Applied Physics Letters, Vol. 44, No. 7, pages 700-702, Apr. 1, 1984, and T. Fukuzawa, "GaAlAs Buried-Multiquantum Well Lasers Fabricated by Diffusion-Induced Disordering", Applied Physics Letters, Vol. 45, No. 1, pages 1-3, Jul. 1, 1984, and are illustrated in FIG. 2. The device is formed on a semiconductor substrate 14, and involves a buried heterostructure in the form of an MQW 16 that is sandwiched between upper and lower cladding layers 18 and 20. As initially formed, the MQW extends laterally across the width of the section shown in FIG. 2. However, the lateral portions 22 and 24 of the MQW are compositionally disordered to intermix the quantum well material with the intervening barrier layers, resulting in an interdiffused material having a higher bandgap energy (E.sub.g) than the original quantum well material, and a lower refractive index. This effectively prevents lateral current spreading from the remaining central portion of the MQW 16 shown in FIG. 2.
The basic MQW structure is well known and is illustrated in FIG. 3a, with the corresponding electron energy bands shown in FIG. 3b (the MQW structure illustrated in FIG. 3a is rotated 90.degree. from that in FIG. 2). The MQW structure consists of alternating layers of III-V compound semiconductor quantum wells 26 and barrier 28s; the total number of wells for laser applications typically ranges from one to about ten. The material selected for the wells 26 has a significantly lower E.sub.g (energy difference between the conductance and valence bands) than does the material selected for the barrier layers 28. A guiding principle for such heterostructures is the choice of a particular fundamental bandgap E.sub.g, and consequently an optical absorption edge, by the deliberate choice of the material parameters. At the heterojunctions the conduction band and the valence band have discontinuities, while the Fermi level coincides on both sides of the junction. Because of such discontinuities, the energy bands near the heterojunction interfaces are bent and local electric fields are created on each side of the junction. Such band discontinuities are utilized for carrier confinement in semiconductor lasers. A more detailed description of MQWs and their application to guided-wave devices is provided in Introduction to Semiconductor Technology: GaAs and Related Compounds, edited by C. Wang, John Wiley & Sons, pages 512-535, 1990. Typical well materials are GaAs and InGaAs, while corresponding barrier materials are AlGaAs and InGaAsP.
If the barrier material is totally interdiffused with the well material of the MQW, the result is a third material that has a higher E.sub.g than the well, but somewhat lower than that of the barrier; the exact level depends upon the relative amounts of each material in the original MQW. This results in the lateral portions 22 and 24 of the original MQW being converted to confinement layers after intermixing, which serve to confine the current to the active lasing region 16. The increase in E.sub.g for the mixed material relative to the well is accompanied by a reduction in the refractive index, again relative to the well, thereby also increasing the optical-mode confinement.
The compositional disordering of the lateral MQW sections may be achieved by various techniques. Originally, zinc was diffused as an impurity into the device over the desired lateral MQW sections, down to a depth at least as great as that of the MQW. This diffusion, indicated by diagonal stripes in FIG. 2, was found to disorder the portion of the MQW into which it extended. Other techniques were subsequently developed, including the diffusion of other dopants or vacancies, and the implantation of various ions to achieve disordering. In the resulting buried heterostructure lasers, carrier and optical-mode confinement in the direction perpendicular to the epitaxial layers is provided by the cladding layers 18 and 20, while carrier and optical-mode confinement in the lateral direction (in the plane of the layers) are provided by the larger bandgap and lower refractive index of the disordered material relative to the well material. The lower refractive index results from the higher concentration of free carriers (from the dopant diffusion or the implantation), and from the absence of the quantum-well effect.
While implantation is mentioned as a possible way to achieve the desired disordering, it is not very practical because of the very high energies required. The active lasing layer (MQW) is buried relatively deep, typically about 1.5 microns. Implantation energies greater than 1 MeV are required to implant to this depth, so diffusion is normally employed. Diffusion, on the other hand, is normally isotropic and spreads laterally, making it difficult to control.
The buried heterostructure laser generally provides better performance than ridge-waveguide lasers, since the active region is enclosed on all sides by larger bandgap energy material. They have a greater bandwidth, which can exceed 15 GHz. On the other hand, buried heterostructure devices are generally less reliable than ridge-waveguide lasers. Although one buried heterostructure device, referred to as the buried-crescent design, exhibits better reliability, its fabrication requires growth by liquid-phase epitaxy and is thus limited by smaller wafer sizes and non-uniformity problems.
The fabrication of buried heterostructure lasers is quite complicated and usually requires several growth steps. As a result, the integration of such lasers with integrated circuit electronics on the same wafer is difficult, and often involves selective area growths. Furthermore, in buried heterostructure lasers formed by compositional disordering the wafer must be heated during diffusion of the dopant that causes the MQW disordering, typically to about 600.degree.-700.degree. C. for a diffusion of zinc, and about 900.degree. C. for a diffusion of silicon into GaAs based materials. These temperatures can spoil certain integrated circuit devices already on the wafer, such as modulation doped transistors, and is another reason why the formation of the laser in the same process with other electronics is normally not done. If implantation is attempted, high temperatures in excess of 800.degree. C. for GaAs based materials are subsequently required to anneal the implant.
Compositional disordering of MQWs using ion implantation at significantly lower temperatures has been reported in a separate context in Anderson et al., "Compositional Disordering of GaAs/AlGaAs Multiple Quantum Wells Using Ion Bombardment at Elevated Temperatures", Applied Physics Letters, Vol. 53, No. 17, pages 1632-1634, Oct. 24, 1988. Disordering of the MQW is achieved by implanting ions into MQW substrates that have been heated to temperatures of 400.degree.-700.degree. C. This simultaneous implantation and annealing allows the temperature to be significantly less than the temperatures required for diffusion or separate annealing. Non-dopant ions can be used, which reduces free-carrier absorption and thus leads to lower threshold currents and higher efficiencies. However, the depth of the active region in the buried heterostructure laser, and the corresponding very high energies necessary to implant to this depth, have made the Anderson et al. technique inapplicable to buried heterostructure lasers on a practical basis.